In the plasma processing of articles such as semiconductor wafers, a common problem is the coupling of electrical energy to the article being processed. Typically, electromagnetic coupling of RF energy into the "source" region of a plasma chamber is employed to generate and maintain a high electron density plasma having a low particle energy. In addition, RF "bias" energy is usually capacitively coupled in the plasma via the article being processed to increase and control the energy of ions impinging on the article.
In a typical high density plasma reactor, the driving point RF "bias" impedance presented by the plasma is very low. To achieve uniform ion energy and flux to the article being processed (typically essential for etching or other plasma processes), uniform coupling of RF "bias" energy through the article being processed to the plasma is required. The article being processed typically is held against some kind of chuck and RF bias energy is applied to the chuck. What is desired is a constant plasma sheath voltage across the surface of the article being processed.
The degree to which such a uniform plasma sheath voltage can be achieved is a function not only of the plasma density uniformity as generated by the plasma source, but is also a function of the impedance per unit area of the plasma sheath adjacent to the article, the impedance per unit area of the article, the impedance per unit area of any gap between the article and the chuck and the impedance per unit area of the chuck.
Besides electrical coupling, the chuck should be tightly thermally coupled to the article being processed. Typically the temperature of the article is a process parameter to be controlled and this normally means removing heat from or adding heat to the article during processing. Heat transfer in a low pressure or vacuum environment such as that used for plasma processing is generally poor. Some means of providing for adequate heat transfer between the article being processed and adjacent surfaces is usually necessary.
Typical prior art chucks mechanically clamp an article to the chuck with a clamp ring applying a holding force at the periphery of the article. The thermal contact between article and chuck is generally insufficient to accommodate the heat load imposed by the plasma on the article. Without some means of improved thermal contact between article and chuck, the temperature of the article may rise out of acceptable limits.
Gas is typically introduced between the article and chuck to enhance thermal contact and heat transfer from the article to the chuck. The gas pressure required is a function of the heat load imposed by the plasma, the desired maximum article temperature, the temperature at which the chuck can be maintained (such as with liquid cooling), the choice of cooling gas and the article/gas and gas/chuck accommodation coefficients (measures of how effectively heat is transferred between a gas and a surface). For biased high density plasma applications, helium gas is used as the cooling gas and the gas pressure required is typically in the 5 to 30 torr range.
For "low pressure" plasma processes (those operating in millitorr pressure range), some means must be provided to allow a significantly higher pressure in the region between the article and chuck with respect to the ambient pressure in the process chamber. In addition, a leak of cooling gas into the process environment may produce undesirable results. Typically some kind of seal, usually an elastomer, is used to allow maintenance of the pressure difference between the two regions.
If the article to be processed is simply mechanically clamped at its periphery to the chuck, and gas introduced between article and chuck, the article will bow away from the chuck due to the pressure difference across the article. If a flat chuck is used on a disk shaped article, a large gap results between the article and the chuck with a peak gap at the center. Under such conditions, thermal and electrical coupling between the article and the chuck are non-uniform. Mechanically clamped chucks typically pre-compensate such article-bowing by attempting to match the chuck's surface to the curvature of the article under stress. Theoretically, this can be done for simply shaped articles (such as disks), but the presence of discontinuities or complex shapes make analytical precompensation impossible, and. trial-and-error is required. Mismatches in curvatures between the article and the chuck result in a variable gap between such surfaces, resulting in non uniform electrical and thermal coupling.
Electrostatic chucks have been proposed to overcome the non-uniform coupling associated with mechanical chucks. Electrostatic chucks employ the attractive coulomb force between oppositely charged surfaces to clamp together an article and a chuck. In principle, with an electrostatic chuck, the force between article and chuck is uniform for a flat article and flat chuck. The electrostatic force between the article and the chuck is proportional to the square of the voltage between them, proportional to the relative permittivity of the dielectric medium separating them (assuming conductivity is negligible) and inversely proportional with the square of the distance between them. Typically for biased-article high density plasma processing applications (such as SiO.sub.2 etching) a cooling gas is required to improve the heat transfer between article and chuck to acceptable levels. Introduction of gas cooling between article and chuck, while required to achieve adequate heat transfer, causes problems with prior art electrostatic chucks when used in biased-article high density plasma applications.
In particular, the requirement of introducing cooling gas in the region between article and chuck requires that some discontinuity be introduced in the chuck surface, typically some type of hole(s) through the chuck to a gas passage behind the surface. The introduction of any discontinuity in the chuck surface distorts the electric field in the vicinity of the discontinuity, making arc breakdown and glow discharge breakdown of the cooling gas more probable. With DC bias applied between an article and a chuck, and RF bias applied to the chuck, gas breakdown becomes probable with prior art electrostatic chucks such as described in U.S. Pat. Nos. 4,565,601 and 4,771,730.
In the '601 patent, a plurality of radial cooling gas dispersion grooves in an upper surface of a plate electrode connect to and extend outwardly from the relatively large upper end of a cooling gas supply pipe extending vertically to the upper surface of the plate electrode. Cooling gas from the supply pipe travels outwardly in the radial grooves and into a plurality circular gas dispersion grooves also formed in the upper surface of the plate electrode coaxial with the gas supply pipe. The upper surface of the plate electrode with the radial and circular patterns of grooves is covered with a thin insulating film upon which the article to be process is placed. The upper open end of the gas supply pipe forms a relatively large discontinuity in the upper surface of plate electrode. The radial and circular grooves are relatively wide and deep, slightly less than the diameter of the gas supply pipe, and form additional relatively wide and deep discontinuities in the upper surface to the plate electrode and relatively deep separation gaps between the plate electrode and the article to be processed. Further, irregularities in the coated surfaces of the grooves produce non-uniformities in gas flow and in the spacing of the article and the plate electrode. Undesired arc and glow discharge breakdowns of the cooling gas would occur if such an electrostatic chuck were employed in a high RF power, high density plasma reactor.
The same problems are particularly inherent in the electrostatic chuck of the '730 patent which includes a central and/or a plurality of relatively large gas feeding tubes to the upper surface of a plate electrode.
What is desired is an electrostatic chuck that can accommodate cooling gas between the workpiece and the chuck and which is designed to avoid gas breakdown even when the chuck is used in a plasma reactor environment including high RF bias power and high density plasma.